The present invention relates generally to ultrasonic welding. More particularly, the invention relates to an ultrasonic rotary horn having tapered axial input sections for the conversion of axial acoustic energy into radial acoustic energy.
Acoustic welding, and more particularly ultrasonic welding, has become increasingly popular in a number of industries. For example, businesses in the textile and personal products industries often manufacture products such as diapers, clothing, etc., that are ultrasonically welded. Ultrasonic welding tools operate under the principle of applying acoustic energy in the ultrasonic frequency range (i.e., at or above 20 kHz) to a horn. The horn vibrates in response to the applied acoustic energy to further produce an output acoustic energy. The output acoustic energy is applied to the materials (typically thermoplastics) being joined by positioning the horn in the vicinity of the parts. The vibrational energy travels through the parts, and at the interface between the parts, is converted to heat. The conversion is due to intermolecular friction that melts and fuses the parts together.
While a number of horn configurations have been developed, it is well known that a good way to attain high quality and high speed ultrasonic welds is to use a rotary horn with a rotating anvil. Typically, a rotary horn is cylindrical and rotates around an axis. The input acoustic energy is in the axial direction and the output acoustic energy is in the radial direction. The horn and anvil are essentially two cylinders positioned close to each other, rotating in opposite directions with equal or nonequal tangential velocities. The parts to be bonded pass between these cylindrical surfaces at a linear velocity which usually matches the tangential velocity of these cylindrical surfaces. Matching the tangential velocities of the rotary horn and the anvil with the linear velocity of the parts can minimize the drag between the horn and the parts.
Rotary horns are therefore typically made up of at least an axial input section and a radial weld section. The input section receives the axial acoustic energy, while the weld section applies the converted radial acoustic energy to the target parts. While the above-described conventional rotary horn is acceptable for some applications, certain important difficulties remain. One difficulty relates to the desire to obtain a high level of amplitude uniformity. Amplitude uniformity is effectively a measure of the percent of the weld receiving the same amount of weld energy. Specifically, amplitude uniformity is determined by measuring the maximum displacement of the external surface of the weld section (i.e., the weld xe2x80x9cfacexe2x80x9d) for a given input excitation. The minimum displacement for the same excitation is also measured, and the ratio of the minimum displacement to the maximum displacement represents the amplitude uniformity. Thus, a rotary horn with an amplitude uniformity approaching one hundred percent would produce very uniform output acoustic energy over its entire weld face. High amplitude uniformity results in more predictable welds and ultimately lower manufacturing costs.
Some conventional rotary horns have an input section that is solid, and has a nodal point. Others have an input section that is hollow and has a nodal plane. All conventional rotary horns, convert axial acoustic energy into radial acoustic energy through the Poisson ratio effect and a number of top and bottom tuning cuts strategically machined out of the weld section. While the tuning cuts allow the conventional rotary horn to achieve acceptable uniformity levels (on the order of 85 percent or greater), this level of uniformity comes at a cost. Specifically, the machining of the tuning cuts is often quite complicated and requires additional tooling and labor.
Another difficulty associated with conventional rotary horns relates to weld area lengths. Essentially, the weld area length is a measure of the amount of product that can be welded in a given pass of the rotary horn. Thus, larger weld area lengths enable greater product throughput and increased profitability. In order to provide increased weld area lengths it is generally necessary to increase the weld area length of the weld section. Many conventional radial horns have. weld area lengths that are limited to approximately 2.5 inches. Using such a horn on an application requiring weld area lengths of approximately 5 inches would therefore require twice as much welding time and labor. It is therefore desirable to provide a rotary horn that has a minimum amplitude uniformity of 85 percent while maintaining a radial weld area length of 5 inches or more.
Yet another difficulty associated with conventional acoustic rotary horns relates to driving and mounting of the horn. For example, many conventional horns have a single input section for receiving the axial acoustic energy. This single input section serves as the only mechanism for driving the horn with the axial acoustic energy and limits the overall usefulness of the horn to half-wave applications. Furthermore, the horn must be mounted on one side of the weld area. When mounted in this fashion, the horn is loaded in the same manner as a cantilever beam. Conventional single mount designs have been estimated to produce weld forces of 150 pounds or less before deflections become too excessive and cause uneven welding. The result is often a lower pressure weld which, in turn, results in less ultrasonic energy being imparted to the target parts. Less ultrasonic energy being imparted to the weld surface ultimately reduces product line speeds. It is therefore desirable to provide a rotary horn that has mounts located on each side of the weld area, in the same manner as a simply supported beam, to increase weld pressure and the transfer of acoustic energy to the welded parts. The simply supported horn can support five times as much force as the cantilever style mounting method for the same deflection. The mounts in the simply supported mounting method could be polar or nodal mounts.
The above and other objectives are provided by an ultrasonic rotary horn in accordance with the present invention. The rotary horn has a first radial weld section, and a first axial input section operatively coupled to the first radial weld section for receiving a first axial acoustic energy. The first axial input section has a first coned section such that the first axial input section and the first radial weld section convert a portion of the first axial acoustic energy into radial acoustic energy. The radial acoustic energy is in phase with the first axial acoustic energy. The use of a tapered geometry for the first axial input section allows the rotary horn to be designed without reliance on the Poisson ratio effect and complicated tuning cuts.
Further in accordance with the present invention, an ultrasonic rotary horn having two polar mounts is provided. The rotary horn includes a first half having a first radial weld section and a first axial input section for receiving a first axial acoustic energy. The rotary horn further includes a second half coupled to the first half. The second half has a second radial weld section and a second axial input section for receiving a second axial acoustic energy. Each half has a coned section such that the halves convert a portion of the first and second axial acoustic energy into radial acoustic energy. Providing multiple polar mounts improves the weld pressure and therefore increases the ultrasonic energy imparted to the materials being welded.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute part of this specification. The drawings illustrate various features and embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.